1. Field of the Invention
The present invention generally relates to measuring soil characteristics and, more particularly, to using measurements of soil conductivity to (a) estimate in real time the suitability of calcium-based soil stabilization methods and (b) estimate the need for laboratory analysis of soil samples.
2. Description of the Prior Art
Stabilization of clay soils at construction sites using low-cost, calcium-based soil stabilization methods such as the application of lime, Portland cement or flyash, is a routine practice to prevent pavement or structural distresses in structures built upon such clay soils. These distresses, which are expensive to remediate, arise because of the presence of expansive minerals such as ettringite and thaumasite in the soil. These minerals are formed in clay soils having significant soluble sulfate (SO4) content when calcium, aluminum, and water are also present. Aluminum, in the form of aluminum silicate (Al2SiO3), is a prevalent constituent of clay soils, and calcium is present in the materials used to stabilize such soils, often in the presence of water. Thus, in the right combinations, these three components (aluminum, calcium and water), plus sulfate compounds present in the soil, may induce the formation of expansive minerals, which can absorb and hold very large amounts of water, swelling up to 250% by volume. Under these conditions, the sulfate-induced distress can cause extensive damage to structures supported on such unstable soils. Therefore, when using lime (CaO), e.g., to stabilize clay soils that have significant soluble sulfates, it is recommended to apply the lime as a lime slurry. The lime slurry (to provide sufficient water for the expansive minerals to form) is applied to the clay soil, followed by sufficient ‘mellowing’ time (typically one to seven days) to pass before compacting of the soil is performed. In this way, the expansion is allowed to reach an equilibrium condition before paving or other structures are constructed on the soil.
Sulfate-induced distress (sometimes called sulfate heave, due to the formation of ettringite) can arise both because of the presence of sulfates in soils treated using calcium-based materials and because of an uneven distribution of expansive minerals in the soil of the construction site being treated. Soil sampling and laboratory analysis of the samples can be used to map a site to determine its composition in order to decide whether to apply soil stabilization to remediate the site. However, such testing is laborious and expensive, involving disturbing the site to take core samples, one sample at a time, handling and analyzing the samples, etc. These disadvantages are magnified by the fact that sulfates tend to occur unevenly in seams in the soil. Thus, routine testing using a practical density of measurements often misses the location and extent of such a seam because of an insufficient number of samples. Such testing may result in treating a much larger area than necessary or in overlooking an area that needs to be treated because it was not discovered, or worse, causing sulfate heave in areas of the site treated with lime slurry that also have an undetected high concentration of soluble sulfates. Yet, increasing the number of samples raises the costs of the survey to locate sulfate-bearing soils that must be treated with soil stabilization techniques.
What is needed is a non-intrusive, real-time method of testing or screening a construction site for the presence of sulfate-bearing soil that will enable a reliable determination of the need for conventional soil stabilization only in those portions of the construction site that need the treatment. Further, the method should enable identification of areas within the construction site in which soil samples must be taken to determine the sulfate concentration thereof and assist in the selection of the best stabilization method for the particular area.